1. Field of the Invention
This invention relates to a control method for an auto focus apparatus, and particularly to an in-focus judging method in so-called overlap servo wherein the accumulating operation of a charge accumulation type sensor is performed even during the time when a photo-taking lens is being driven to an in-focus position, and in-focus control is effected while new in-focus positions are calculated one after another on the basis of the output of the sensor.
2. Related Background Art
FIG. 4 of the accompanying drawings shows a general block diagram of an auto focus apparatus in which a photo-taking lens is driven under servo control by a motor and thereby brought into an in-focus state. In FIG. 4, a light beam from an object transmitted through a photo-taking lens 41 responding to the auto focus apparatus is imaged on a light receiving sensor 42 which is in-focus state detecting means provided in a camera body, and an optical image signal from the light receiving sensor 42 is sent through an interface 43 to a controller 44 for controlling the entire system. Usually, a light receiving element of the charge accumulation type such as a CCD is used as the light receiving sensor 42 and a microprocessor is used as the controller 44. A microprocessor is also called a microcomputer, but for simplicity, the controller will hereinafter be referred to as CPU. The optical image pattern on the light receiving sensor 42 is A/D-converted by the interface 43 and output to the CPU 44, or is amplified into a suitable signal level by the interface 43 and directly A/D-converted by an A/D converter contained in the CPU 44.
The optical image pattern converted into a digital signal in this manner is data-processed by the CPU 44 in accordance with a predetermined algorithm to thereby calculate the amount of movement of the photo-taking lens 41 necessary to bring about the in-focus state. This will hereinafter be called the defocus amount. Herein, the optical principle and algorithm for the calculation of a specific defocus amount need not be described becausse there are already many known examples thereof.
The photo-taking lens 41 is provided with an encoder 46 to monitor the movement thereof, and each time the photo-taking lens 41 is moved by a predetermined amount along the optical axis thereof, the encoder 46 generates a pulse Pf. Hereinafter, this will sometimes be called the feedback pulse Pf. The CPU 44 indicates the calculated defocus amount (the amount of movement of the lens) to a motor driver 45 and drives a servo motor 47, thereby driving the photo-taking lens 41 in the in-focus direction. The movement of the photo-taking lens 41 is monitored by the CPU 44 with the aid of the feedback pulse Pf from the encoder 46, and when the pulses from the encoder 46 are counted by a pulse number corresponding to the defocus amount, the driving of the motor 47 for driving the photo-taking lens is stopped. Usually, the encoder 46 is constructed by a photointerrupter or the like being attached to a portion of the rotary shaft or the reduction gear of the motor so as to detect the rotation of the motor 47 for driving the photo-taking lens.
FIG. 5 of the accompanying drawings illustrates the defocus amount detected by the automatic focus adjusting apparatus. In FIG. 5, the defocus amount is defined as the distance between a position at which the light beam transmitted through the photo-taking lens 41 is imaged and the surface of film, i.e., the image plane deviation amount .DELTA.Z. That is, a case where the imaging plane of the photo-taking lens 41 is on the film surface fo refers to the in-focus state, a case where said imaging plane is on f.alpha. refers to the so-called front focus state, and a case where said imaging plane is on f.beta. refers to the so-called rear focus state. Also, as is apparent from FIG. 5, if the object is relatively far, the defocus amount .DELTA.Z is substantially equal to the amount of movement of the lens necessary to achieve the in-focus. Accordingly, to cause an optical image to be formed (focused) on the film surface, the photo-taking lens 41 can be driven back and forth by the defocus amount .DELTA.Z.sub..alpha. when the lens is in the front focus or the defocus amount .DELTA.Z.sub..beta. when the lens is in the rear focus state.
In this sense, in the description of AF servo made with reference to FIG. 4, the amount of movement of the photo-taking lens 41 necessary to achieve the in-focus state thereof is also defined as the defocus amount. Exactly, the defocus amount .DELTA.Z and the amount of lens driving do not coinside with each other, but in the description of the present invention, it is considered that they are equal to each other.
The purpose of auto focus is to bring a photo-taking optical system into the in-focus state and generally, to detect this, it is judged when the distance measurement output of an AF sensor is smaller than a certain threshold value that the photo-taking optical system is in focus. The inside of this threshold value is called "the in-focus zone", which is determined with the open F-value or the like of the photo-taking lens taken into account, thereby avoiding that unnecessarily strict accuracy is set to aggravate the convergence of servo. There is also a method whereby a second threshold value greater than another in-focus zone is provided discretely and when the calculated defocus amount is smaller than this second threshold value, after the termination of the lens driving under servo control based thereon, it is inferred that the lens is in the in-focus state before distance measurement is effected again. This servo method is called "open loop servo", and the second threshold value is called "the open loop zone". According to this, there is the advantage that an in-focus signal can be put out without awaiting the confirmation of the in-focus by the next distance measurement, and the disadvantage that the photographer is made to wait for some time until an in-focus signal is put out after the termination of the movement of the lens can be made inconspicuous. This method is greatly effective perticularly when the accumulation time is long. Putting out an in-focus signal specifically means effecting the display of the in-focus, or putting out a release permission signal in the case of the in-focus priority photographing mode.
FIG. 6 of the accompanying drawings is a graph showing the conventional servo control of auto focus, and the abscissa of this graph represents time and the ordinate of this graph represents defocus amount. In FIG. 6(a), the portion indicated by hatching is the sensor accumulation time. IN the conventional AF servo, distance measurement and lens driving are effected sequentially and therefore, during distance measurement, the lens is stationary as shown in FIG. 6(a). In this example, at first, distance measurement was effected in an accumulation time t.sub.1 and defocus Z.sub.1 was detected, and on the basis of the result of this, the lens was driven under servo control, but when distance measurement is again effected thereafter in an accumulation time t.sub.2, defocus Z.sub.2 is detected and further on the basis of this, the lens is driven under servo control, whereafter defocus Z.sub.3 obtained by effecting distance measurement again in an accumulation time t.sub.3 is judged to be smaller than the in-focus zone and lens driving is not effected. FIG. 6(b) shows the timing at which the in-focus signal when the judgment of the in-focus has been so done is output. Also, when in FIG. 6(a), the defocus amount Z.sub.2 by the second distance measurement is smaller than the open loop zone, an in-focus signal is output at the timing of FIG. 6(c).
FIG. 7 of the accompanying drawings is a graph showing the operation of overlap servo. The photo-taking lens is driven in the in-focus direction with time, and the defocus amount decreases from one moment to the next and moves as indicated by the curve in FIG. 7(a). The period indicated by hatching from a time t.beta. to a time t.beta. is the accumulation time of the AF sensor which overlaps with lens driving. The fact that accumulation and lens driving overlap with each other means that although not shown, the servo control by the last distance measurement has continued hitherto. The count values of feedback pulses at the time t.alpha. and the time t.beta. are P.alpha. and P.beta., respectively. When accumulation is terminated at the time t.beta., although not described in detail, the average distance measuring position corresponding to a crude defocus amount Zm calculated from the then sensor output by the overlap servo control method previously proposed by the applicant (Japanese Laid-Open Patent Application No. 2-146010) is calculated as a value Pm converted in terms of the count value of the feedback pulses. As a matter of course, Pm is a value between the count value P.alpha. and the count value P.beta.. The feedback pulses when the movement of the lens was monitored are shown in FIG. 7(b). The amount of movement of the image plane per one of these pulses is substantially the same for each lens and is integrated by a counter and indicates the amount of movement of the lens. The counter value does not indicate the absolute position of the lens, but indicates the relative position of the lens with the pulse count as a unit. After the accumulation by the sensor has been terminated, a processing time (called the algorithm time) is necessary to process the sensor output and calculate the defocus amount. In FIG. 7, the period indicated by the time from a time t.beta. till a time tc corresponds to the algorithm time. Even during the algorithm time, the lens driving by servo control based on the last defocus amount is continued and therefore, even if calculation is terminated at the time tc and the defocus amount Zm is obtained, it is necessary to subtract the defocus amount Z (Pc-Pm) corresponding to the difference (Pc-Pm) between the average distance measuring position Pm represented by the pulse count and the count value Pc at the time tc and make the result a servo control target amount. Here, the defocus amount Z(p) indicates a function for converting the pulse number p into a defocus amount Z, and hereinafter (Pc-Pm) or Z(Pc-Pm) will be called "the amount of correction". When the luminance of the distance measuring area is high and the accumulation time is short, the distance by which the lens is moved in the meantime is small and therefore, the pulse number (Pc-Pm) has been created almost during the algorithm time of AF, but in contrast, when the luminance is low and the accumulation time is long, the distance by which the lens is moved during the accumulation time is great and therefore, the difference between the average distance measuring position Pm represented by the pulse count and the count value P.beta. at an accumulation termination time t.beta. becomes great. The algorithm time depends on hardware which carries out the calculation process, and usually a micro-computer is used for this purpose and the processing time is changed by the object image, but the change is much less than the range of change of the accumulation time.
However, the application of the conventional in-focus judgment method to the overlap servo previously proposed by the applicant and schematically described above would encounter problems as will hereinafter be discussed.
In the structure of the overlap servo as previously described, it is at the time tc that the result of the calculation of the defocus is put out, and what is the subject of the in-focus judgment when effected is not the crude defocus amount Zm calculated by processing the sensor output, but the crude defocus amount minus the amount of correction Z(Pc-Pm). However, even if Zm-Z(Pc-Pm) indicates the inside of the in-focus zone, there may be a case where the crude defocus amount Zm by the sensor output is considerably greater than the in-focus zone, but nevertheless the amount of correction Z(Pc-Pm) happens to be approximate to Zm and therefore the corrected defocus amount Zm-Z(Pc-Pm) is within the in-focus zone and this is judged to be the in-focus. In such case, the distance measurement error included in the crude defocus amount is not negligibly small and therefore, actually it is often the case that the crude defocus amount is not within the in-focus zone, and misjudgment is apt to occur. Further, if the accumulation time is long and the amount by which the lens is moved in the meantime is great relative to the defocus amount, the optical image of the object on the sensor will change from one moment to another in conformity therewith, and the sensor output read out at the accumulation termination time t.beta. will become low in contrast and the defocus amount Zm obtained by this signal being data-processed will be of bad accuracy. Generally, it is unavoidable that the more the lens is moved during the accumulation, the more the accuracy of distance measurement is reduced, but unless this is considered to be one of judgment elements for the in-focus judgment, it will often be the case that in spite of the lens actually being not in focus, the lens is judged to be in focus.
In order to avoid these problems, there would occur to mind a method whereby the judgment of the in-focus is made when the crude defocus amount Zm itself is within the in-focus zone and moreover the defocus amount Zm-Z(Pc-Pm) corrected by subtracting the amount of correction Z(Pc-Pm) is also within the in-focus zone. In terms of accuracy, this method could be sufficiently reliable, but satisfying this judgment condition means that the lens is hardly being moved during accumulation and during the algorithm time after that, and actually, it is a case where lens driving is terminated and the result obtained by effecting distance measurement again thereafter is within the in-focus zone (FIG. 8 of the accompanying drawings) or a case where accumulation is started near the servo target and the accumulation is terminated after the termination of driving (FIG. 9 of the accompanying drawings). In such a case, if the accumulation time is long, it is after a considerable time has passed after the termination of the driving of the lens that the in-focus judgment can be made, and this has led to the disadvantage that responsiveness is reduced.